1. Field of the Invention
The present invention relates generally to electrical initiation systems for explosive devices and, more particularly, to low impedance slapper detonators and feed-through assemblies for use in initiation systems of explosive devices such as, for example, advanced warheads.
2. Discussion of the Related Art
The explosive charge in a slapper detonator is initiated by a flyer plate which is driven to a very high speed over a short distance by the hot gaseous metal and plasma generated by passing a very high current pulse through an exploding foil initiator (EFI) having a thin foil or film bridge. The term "exploding" is used because of the required speed of transformation of the bridge from solid metal to super heated metal vapor and plasma. If the bridge is heated too slowly, it can melt and perhaps electrically open as does a common electrical fuse. Heating the bridge too slowly can also melt, blister, or otherwise damage the material used as the flyer plate. In order to successfully initiate an EFI, it is important to heat the bridge to its melting point before the flyer plate has been disturbed. If the bridge is heated quickly enough, the power in the high current firing pulse flowing through the plasma from the bridge material (which is typically measured in megawatts) goes directly into accelerating the flyer plate material away from the bridge area at an extremely high rate. In other words, the current through the bridge must be as high as possible by the time the total energy dissipated in the bridge is high enough to cause it to burst.
A very rapid rate-of-rise of the current pulse into the initiator, e.g., on the order of tens of thousands of amperes per microsecond, is desirable in order to achieve suitable acceleration of the flyer plate; however, the rate-of-rise of the current pulse is limited by the inductance of the initiating system. Inductance is the physical property of electrical circuits which acts to directly oppose changes in current flow. The voltage drop across an inductor is simply its inductance times the time rate of change of current (L di/dt). For a hypothetical initial firing voltage of 1,400 volts, a 20 thousand V/.mu.s rise requires an inductance of no more than 70 nanohenries (nH).
The simple formula set forth above can only predict the initial slope of the current pulse. The effective average rise until the bridge bursts is usually somewhat lower. The formula can, however, be used to accurately estimate the actual inductance of an initiator system from the initial slope of a firing current trace. For a typical EFI and firing circuit, a total system inductance of 20 nH or less is a more reasonable design target.
EFI's for slapper detonators have often been formed by etching a foil bridge on flexible printed circuit boards made of polyimide and placing a hollow, cylindrical member or barrel on a side of the printed circuit board opposite the bridge to create a gap across which the polyimide material can accelerate before impacting an initiating explosive on the other side of the barrel. A number of different approaches have been developed to connect such EFI's to fuze electronics and other sources of current. One approach involves laying the EFI across a pair of layered striplines which are longitudinally offset from one another such that one end of the bridge contacts a copper strip on the bottom stripline while the other end of the bridge contacts a copper strip on the top stripline. The striplines are then clamped or soldered to the fuze electronics to complete the connection. Another approach involves folding the flexible printed circuit board such that the bridge is disposed on an inner surface thereof and providing terminal ends of the boards with holes surrounded by concentric contact rings in the form of a bulls eye so that the printed circuit board can be electrically coupled with the fuze electronics using a bolt inserted through the holes. Yet another approach using flexible printed circuit board technology involves use of a contact board having a coaxial connector thereon with inner and outer contacts configured to mate with the fuze electronics and a transition board disposed between the EFI and the contact board to connect opposite ends of the bridge with inner and outer contacts of the coaxial connector via plated through-holes formed in the boards.
A disadvantage of these approaches is that they are difficult to incorporate into a hermetic package without incorporating the EFI and fuze electronics in a single sealed package which would preclude periodic testing of the fuze electronics by preventing disassembly. An approach achieving hermeticity has been developed utilizing a folded flexible printed circuit board with a bulls eye connector disposed between a barrel and a glass-to-metal sealed feed-through. The feed-through includes pins embedded within glass and contacting portions of the EFI; however, in order to match thermal coefficients of the glass and the metal contacts, it is necessary to use exotic metals such as Kovar which are not highly conductive and, therefore, can increase the inductance of the system. A disadvantage of detonators utilizing flexible printed circuit boards in general is that the thicknesses of the polyimide material and copper cladding are fixed thereby limiting the bridge and flyer plate design to commercially available thicknesses which may not be optimal.
Exploding foil initiators have also been formed using silicon chip technology by depositing a layer of metal on a silicon or ceramic substrate to define a bridge and covering the bridge with an oxide layer which functions as a flyer plate. These so-called bridge chips are advantageous in that the thickness of the bridge and flyer plate materials can be tailored for the particular application; however, connecting firing electronics to such chips can be difficult. One approach involves mounting the chip on a carrier with plural J-shaped leads configured to mate with the firing electronics and connecting the bridge contact pads with the leads using fine wires. While this approach is widely used in commercial applications, there are certain disadvantages when using this technique to build a slapper detonator.
Firstly, the use of fine wires tends to increase the inductance and resistance of the initiating system thereby increasing overall energy requirements. Inductance can be lowered to suitable levels by using a large number of wires, however, connecting a large number of fine wires in this manner is a delicate procedure requiring complex and costly equipment. The presence of a large number of wires also makes it difficult to position a barrel immediately adjacent the bridge. Another approach involves drilling holes through the chip, plating the holes and using the plated through-holes to connect to a stripline or printed circuit board of the firing electronics. While this approach reduces inductance and permits placement of a barrel directly over the bridge, it is difficult and costly to form through-holes in a silicon or ceramic substrate. Still another approach involves separating the copper strip from a stripline of the firing electronics and bending the copper strips so that they overlap the contact pads on the chip. Some of the disadvantages of this type of approach are that it is difficult to assemble and can result in higher inductance due to bending of the copper strip. An additional disadvantage of the aforementioned exploding foil initiators with bridge chips is that the exploding foil initiators are soldered to the firing electronics and thus are not easily removable therefrom to permit periodic testing of the firing electronics.